1. Field of the Invention
The present invention relates to a silica glass crucible having high purity in its inner portion, and its production method, wherein said silica glass crucible is used for pulling a single silicon crystal being used to a semiconductor etc. from the melted silicon. In addition, the present invention also relates to a pulling method using said silica glass crucible.
2. Discussion of the Prior Art
A little amount of alkali metals has been contained in natural silica. When the silica glass containing said alkali metals is used as a heat treating substance for semiconductor industries etc., the substance is deformed or devitrified easily. Moreover, in the case of using for the silica glass crucible for pulling the single silicon crystal, there is a problem that the quality of the single silicon crystal is affected with the impurities, such as alkali metals etc., being contained in the crucible.
As means for solving such problems, it has been proposed that the silica crucible, in which the inside of the crucible is formed with the high purity synthetic silica glass layer and the outside of crucible is formed with the natural silica having a comparatively high strength. However, since the temperature and using time of the crucible at the time of pulling are under a severe condition by the enlargement of the pulled single crystal, there are new problems that the alkali metals etc. being contained in the natural silica glass layer in the outside of the crucible is diffused to the synthetic silica glass layer in the inner and dissolved to silicon melt, to give bad effects to the quality of the single silicon crystal. On the other hand, when the whole of the crucible is made with the high purity synthetic silica glass, the problem about the impurity diffusion is solved, but in the present condition, since the synthetic silica glass has very lower viscosity than that of the natural silica, the whole of the crucible made with the synthetic silica glass is deformed or buckled easily, so that said crucible can not be used for long time.
Moreover, for requiring the high performance silica glass crucible, it has been in the tendency that the temperature for melting the glass becomes high. When the temperature becomes high, the viscosity is decreased to vent air bubbles easily and a concentration of OH groups of the silica glass becomes low. As the result, there are problems that the impurity is concentrated on the inside surface of the crucible by the vaporization of SiO, and the impurities, such as alkali metals etc., is diffused from the natural layer in the outside of the crucible to the high purity synthetic layer in the inside of crucible during the arc melting, as described above. When HF etching is carried out to remove such impurities being contained in the inside surface of the crucible, it takes time very much, and in addition, an irregularity, which is said to as HF cracks in general, is generated on the surface by the HF washing for long time, so that the yield of the single crystal becomes to be decreased on the contrary.
[Problems to be Solved]
In such a situation, as for the produced silica crucible or the producing silica crucible, it has been proposed that the method, in which the alkali metals etc. are moved to the outside of crucible by applying the voltage to decrease the concentration of these metals in the inside of crucible (Japanese Patent Publication No. H07-84328). However, since a plasma atmosphere is occurred during an arc discharge at the time of producing the crucible, the discharge is generated violently around an electrode when the electrode is charged in positive. In addition, from the reason, such as the insulation between the arc plasma and the mold is not sufficiently, it is difficult to carry out the electrolytic refining sufficiently. By the way, in the case that the silica powder is filled in the mold to be heated and melted to make the crucible, since the upper end of the mold and the upper end of the crucible are the same height, it is difficult to take an electrical isolation between the arc plasma and the mold in this configuration.
In addition, the following methods and device have been also known. That is, the method comprises, heating the melted silica glass, applying direct current voltage of from 10 to 50 KV to said heated glass, and removing the alkali metals or Cu (Japanese Patent Publication No. H07-014822). The method comprises, pulverizing a natural quartz crystal, refining it, carrying out the arc melting to form the crucible, and applying the high voltage to said crucible (Japanese Patent Publication No. H07-080716). The device comprises, applying the voltage to the silica glass formed body and removing the impurities (Japanese Patent Publication No. H06-104577). However, all of these methods in the above examples are aimed at the crucible being formed already, so that they are not the electrolytic refining during the arc melting.
Moreover, the method for applying the voltage between the crucible and the positive or negative electrode is not described in said conventional methods. However, in the method, in which the crucible formed body is contacted with the positive or negative electrode and applied by the voltage, it is difficult to carry out electrolysis because of the following reasons.                (A) When the metal plate is used as the contact electrode, the impurity contamination is caused to decrease the quality and the dislocation free ratio of the single silicon crystal.        (B) When the powder, such as carbon etc., is used as the contact electrode, the carbon powder is reacted with the silica glass and its surface becomes an irregular under the temperature more than 1100 degree C., so that the dislocation free ratio of the single silicon crystal is decreased.        (C) In the conventional method, in which the voltage is applied to the silica glass formed body, the voltage applying is carried out at the temperature not deforming the crucible, i.e. less than 1400 degree C., and when the silica powder is used as the negative electrode and the crucible has the silica powder layer of form 5 to 30 mm, the applied voltage is almost inclined to the silica powder layer, so that the alkali metals etc. in the inside of crucible is not moved. At this time, in order to carry out the economical electrolysis, it is necessary to apply the voltage at least 30000 V, and it is very difficult technically to insulate such a high voltage at the high temperature.        